A grain oriented electrical steel sheet is mainly used as an iron-core material for electric devices such as a transformer and so forth. Therefore, there is a demand for a grain oriented electrical steel sheet having an excellent magnetization property, in particular, low iron loss.
Such a grain oriented electrical steel sheet is manufactured, for example, by performing hot rolling on a steel slab containing an inhibitor (source thereof) necessary for secondary recrystallization, for example, MnS, MnSe, and AlN, by performing hot band annealing as needed, by thereafter performing cold rolling one time, or two times or more with intermediate annealing interposed between the performances of cold rolling to obtain a final thickness, thereafter performing decarburization annealing, thereafter applying an annealing separator such as MgO to the surface of the steel sheet, and thereafter performing final finishing annealing. A forsterite (Mg2SiO4)-based insulation coating (forsterite layer) is formed on the surface of such a grain oriented electrical steel sheet other than in exceptional cases.
The forsterite layer effectively contributes to a decrease in the amount of eddy current by electrically insulating stacked steel-sheet layers from each other when the steel sheets are used in the form of stacked layers. However, when the forsterite layer on the surface of a steel sheet is non-uniform or when the flaking of the forsterite layer occurs when a wound core is manufactured, there is a decrease in commodity value. In addition, there is a decrease in lamination factor, and also local heat generation occurs due to a decrease in insulation property which is caused by pressure generated when the wound core is assembled, which results in an accident in a transformer.
In addition, such forsterite layer is not formed only for the purpose of electric insulation. Since it is possible to give tensile stress to a steel sheet by utilizing the low thermal expansion of a forsterite layer, a forsterite layer contributes to improvement in iron loss and, further, in magnetostriction. Further, such forsterite layer contributes to an improvement in magnetic property through purification of the steel by absorbing the constituents of an inhibitor which are no longer needed into the forsterite layer when secondary recrystallization has been completed. Therefore, obtaining a uniform and smooth forsterite layer is one of the important points influencing the product quality of a grain oriented electrical steel sheet.
In addition, when the amount of forsterite is excessively large, point-like defects in which flaking occurs locally in a forsterite layer, tend to occur in general. On the other hand, when the amount of forsterite is excessively small, there is a decrease in adhesion property with, for example, a steel sheet. To date, therefore, the amount of forsterite formed (the amount of forsterite) and the distribution morphology of forsterite have been given importance. In addition, since it is necessary to control the amount and distribution morphology of forsterite to manufacture a grain oriented electrical steel sheet, it is very important to evaluate these factors.
Examples of techniques for investigating the amount of forsterite and the distribution of the amount of forsterite include the following. One is a method in which the amount of forsterite is determined by performing oxygen analysis on the surface of a steel sheet. Specifically, since a tension coating layer is usually formed on a forsterite layer to further improve the magnetic properties, this tension coating layer is removed first, then, steel is dissolved and, then, the amount of oxygen is determined using an infrared absorption method after combustion.
In addition, examples of methods of checking the distribution of a forsterite layer include one in which a surface from which the tension coating layer has been removed is observed using a scanning electron microscope (SEM). In that case, elemental analysis may be conducted by detecting characteristic X-rays.
In addition, examples of methods of investigating the distribution in a cross section include one in which the cross section of a steel sheet is prepared by performing, for example, polishing and in which the cross section is observed using a SEM (for example, Japanese Unexamined Patent Application Publication No. 2012-36447).
However, the methods described above all involve destructive analyses. In addition, any of such analyses takes a long time to complete the evaluation and prepare samples. Moreover, there is currently no method of even checking the presence of forsterite easily without destroying a measurement object.
It could therefore be helpful to provide a technique to easily check the presence of forsterite without destroying the measurement object.
In addition, it could be helpful to provide a technique to easily check the location where forsterite is present without destroying the measurement object.
In addition, it could be helpful to provide a technique to check the amount of forsterite and distribution of the amount of forsterite in a non-destructive manner, in a field of view wide enough to represent the object, and quantitatively.